This invention relates generally to a heat actuated fuse and to an apparatus which incorporates such a fuse for protecting electrical circuitry. The invention is particularly useful for interrupting current flow to thick film circuits and components.
A heat actuated fuse has an important advantage over the more common current actuated fuse. Although both types of fuses ultimately open at a predetermined temperature, a heat actuated fuse can be positioned such that it responds directly to the temperature of a protected component or circuit, and temperature is often a better indicator of imminent failure than current. Examples of known heat actuated fuses are described in Great Britain Patent 2,145,295A and U.S. Pat. Nos. 4,533,896 and 5,097,247.
Great Britain Patent 2,145,295A discloses a thermal fuse disposed on a substrate nearby a resistor, which is also disposed on the substrate. The fuse includes a pair of electrodes defining a gap between them, a gold fuse link extending across the gap to electrically interconnect the electrodes, and a film of solder overlying the gold fuse link and overlapping at least one of the electrodes. When the temperature of the substrate exceeds the melting point of the solder film, the solder is intended to melt, dissolve the gold fuse link, and then retreat from the gap with the dissolved gold to sever the electrical connection between the electrodes.
This fuse is relatively simple to manufacture, but its reliability depends on thorough dissolution of the gold fuse link by the process of leaching. This is a gradual process which begins after the film of solder melts. The gold fuse link then dissolves and is attracted to tin in the solder. The speed of opening of this fuse is relatively slow, and the solder material is limited to a composition containing tin or whatever material will effectively leach the gold or other metal chosen for the fuse link.
U.S. Pat. No. 4,533,86 discloses a fuse for protecting thick film devices deposited on a substrate. The fuse includes two terminal clocks, each including a hole, which are mounted to the substrate in close proximity to each other and with the holes aligned. An electrically conductive fusible link (e.., solder) is suspended across a space between the terminal blocks and extends into the holes to complete an electrical circuit. The holes are larger than necessary to accept the fusible link so that, when the fusible link melts because of excessive heat, molten material from the fusible link will be drawn into the holes.
Manufacture of this fuse is a complicated and labor intensive process. Manufacture requires boring of holes in the terminal blocks, coating the inner surfaces of the holes and the end surfaces of the terminal blocks with solder, assembling the blocks and fusible link, and then positioning and soldering the assembled fuse onto electrodes provided on the substrate.
U.S. Pat. No. 5,097,247 discloses an arrangement of materials which function as a fuse and can be deposited on an electrically insulating substrate, using standard printed circuit fabrication techniques. First and second electrodes are disposed on the substrate, defining a gap therebetween. A layer of first solder paste material, having a melting temperature higher than the actuation temperature of the fuse as disclosed on at least one of the electrodes. A conductive link disposed on at least a portion of each electrode and on the substrate extends across the gap to provide an electrical connection between the electrodes. The conductive link is comprised of a second solder paste material which has a melting temperature substantially corresponding to the actuation temperature of the fuse.
The melting temperature of the second solder paste, however, necessitates an inert, nitrogen or otherwise non-oxidizing atmosphere during manufacture. Depending upon the circuit board to which the fuse is attached and how the board is processed, the paste may reflow and inadvertently open the fuse.
Accordingly, there are a number of problems and drawbacks to the present technology. There is a need for a fuse and/or a method of manufacture to meet the requirements described above, as well as others known to those skilled in the art.